samiam
Punkic Cyborg
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Post by samiam on Jan 19, 2020 3:51:05 GMT
There is this old design, and an LM1881 ought to work, too, but is anyone using anything cheaper/simpler? With a binary signal like this, it seems like a very cheap op-amp ought to do the job. Thanks.
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keithcourage
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Post by keithcourage on Jan 20, 2020 3:53:24 GMT
Are you talking about what to use with RGB for the sync signal?
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samiam
Punkic Cyborg
Posts: 100
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Post by samiam on Jan 20, 2020 5:02:59 GMT
Specifically, I'm wondering how people who are tapping pin C22 on the expansion connector (which corresponds to pin 44 of the HuC6260) for sync are dealing with the signal. I'm pretty sure it can't supply enough current to be used on its own. This is for that PCE whose luma pin I fried; I need to do an RGB mod to salvage it, and I can't use composite video for sync, with a sync stripper or otherwise, because the sync pulse there was originating from luma.
I measured pin C22 on my scope and was surprised to see that the signal is 0.725Vp-p with the positive peak at 5V DC. So the good news is, it doesn't need amplifying, but the bad news is, it has a huge DC bias.
I think I'm going to use a comparator. That means few parts, few connections, little power draw, and the ability to easily modify it to drive 1Vp-p or greater if I ever need it.
Thoughts?
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keithcourage
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Post by keithcourage on Jan 20, 2020 22:27:20 GMT
Tap pin 44, same as C22 that you mentioned, and add a capacitor in line (220uf or 330uf is fine.) and run to the Composite/sync pin of the RGB port. Simply cut the trace for composite so both signals are not feeding into one another(needed for DUO AV port only). I usually just mount the capacitor right to the AV port itself. This is basically the same idea as Voultars amp but he has the capacitor for sync mounted on his board instead of a cap directly on the AV port.
I suppose you could even just put the capacitor for sync inside your scart cable instead.
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samiam
Punkic Cyborg
Posts: 100
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Post by samiam on Jan 20, 2020 23:27:07 GMT
Oh wow, so it can source the current as long as you remove the DC bias? Comparators are so simple and so cheap, I'll probably try one anyway, but it's good to know that I can just use a cap if I need to. Thanks! EDIT: Hmm. Voultar's newer amp design has the signal buffered. voultar.com/index.php?route=product/product&product_id=53
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keithcourage
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Post by keithcourage on Jan 21, 2020 2:15:18 GMT
Ah okay, I'm used to the older design that just has a cap on the board. Either way, most people have just been tapping the signal and placing a cap in between for years with no ill effects. Slightly better picture this way over using composite video for the sync as well. I still prefer to use a common 7314 RGB amp design.
You don't even have to cut the trace for composite if you are just installing your own AV/RGB port. The only reason to cut the trace for it on a DUO is so the two signals don't go together since the sync pin on the AV port is the same as composite on a DUO board. When I install my own AV/RGB port in PCE, I simply run a wire from the expansion port pin for sync to a 220UF capacitor with the negative end soldered directly to the sync pin for the AV/RGB port. Then wire up the ground, 5V, and RGB lines to the AV port as usual.
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samiam
Punkic Cyborg
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Post by samiam on Jan 21, 2020 2:54:54 GMT
That's very interesting. I might throw a cap and a 75 ohm resistor to ground on a board just to measure the signal under load, but I believe you that it works.
0.725V into a 75 ohm load draws just shy of 10mA, which does strike me as a lot to come from this one pin. For someone with a monitor that accepts 0.3Vp-p sync (which most do) I might add a resistor after the cap to reduce that current.
Anyway, I'll let you know how the comparator works. It should just take the IC and three resistors. I'll also be using a 7314 amp myself.
Thanks!
By the way, have you ever made an audio amp for white PCEs or the US TG16?
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keithcourage
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Post by keithcourage on Jan 21, 2020 3:22:14 GMT
I've never bothered to build one but here is a good known working design. oshpark.com/shared_projects/uFE90NazInterestingly enough, there is also a pretty simple one found on that same component video I got here.
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samiam
Punkic Cyborg
Posts: 100
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Post by samiam on Jan 21, 2020 3:33:20 GMT
Thanks again. I just found this. The amp in your pic is very similar to the one on the left. I'm going to try to get a 7314-based RGB amp, a comparator-buffer for sync, and an audio amp all on one board that will fit inside a white PCE. The audio amp I use will probably depend on how much space there is more than anything, really.
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keithcourage
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Post by keithcourage on Jan 21, 2020 3:39:16 GMT
Interesting, did you see there is also a RGB to S-video conversion circuit up higher in the same thread?
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samiam
Punkic Cyborg
Posts: 100
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Post by samiam on Jan 21, 2020 4:00:14 GMT
Ah, it's not RGB to S-Video, but Y/R-Y/B-Y to S-Video.
I had seen it before. The thing is, this guy states he didn't test it. While I didn't build this exact circuit, I did try biasing the output of pin 40 in a similar way, and it didn't work.
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keithcourage
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Post by keithcourage on Jan 21, 2020 4:11:30 GMT
Oh okay, I didn't notice the Ys in there when I glanced past it. Yeah, I seem to find a lot of diagrams online for various video upgrades where people say, this circuit should work but I've never built or tested it.
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